Sv: Re: Saab 99 Bosch K-jetronic rally engine.

Forum Övrigt SSR Saab 99 Bosch K-jetronic rally engine. Sv: Re: Saab 99 Bosch K-jetronic rally engine.

turbo killer

So maybe there is not much information about the works rally cars that used K-jet? Although i have since been informed about Per Eklunds 1978 Swedish Rally car (HGF641) that is reported to have 155bhp, although i do not have any details about the engine or if the K-jet was standard or modified or even from another car? Maybe Peter from the museum will have more information?

When i finally finished building my ’fast road’ engine i took it to the dyno i always use which is quite accurate and consistant. The specification of the engine is as follows;
1. H series gas flowed head with 44mm inlet valves and standard injection exhaust valves.
2. Camshaft from Catcams, special profile 284*@0.1mm lift and 256*@1.0mm lift, the total lift is 12.2mm and the Lobe centre line angle is 109* (fairly mild cam) with adjustable sprocket.
3. K-jet from a 99 turbo and adjustable warm up regulator.
4. MSS 4-2-1 exhaust manifold attached to a 2” Jetex system.
5. Standard ignition.

The cylinderhead and advice was from my good friend Dave Baker at Puma Race Engines

We carried out a few power runs on the dyno to get things fully warm. The engine starts to pick up from 2000rpm and climbs well to 5000rpm it then stops climbing and the line stays flat all the way to over 6500rpm, this peak is at 140bhp. At first we thought it could be a restriction with the exhaust, because this is what happens with an exhaust restriction, but this type of system has been used on other cars with higher power and no such symptoms. So next we fitted a calibrated vacuum gauge to a nipple on the inlet manifold to see what was happening at wide open throttle, the result was quite surprising, the manifold contained a vacuum of between -0.2bar and -0.3bar, i gave this information to Dave Baker and he explains what is happening very well;

In an ideal world a perfect induction system would have almost zero manifold depression at WOT. Any restriction there means a reduction in the amount of airflow into the engine. When American carb manufacturers started flow testing their carbs to give a CFM (cubic feet per minute) rating they settled on a flow bench test pressure of 1.5″ of mercury. The reason was that it had been found that if manifold depression exceeded that amount power started to suffer. In other words their flow figures were saying you can use this carb to supply that number of CFM of airflow and it won’t cause a manifold depression greater than 1.5″ of mercury.

Atmospheric pressure (14.7 psi) is about 30″ of mercury so 1″ of mercury is about 0.5 psi. 1.5″ of mercury is therefore about 0.75 psi and that’s as much as we ideally want to see at WOT. 0.2 to 0.3 bar is a huge manifold depression. 1 bar is 14.5 psi so we’re looking at 2.9 to 4.35 psi. That’s reducing the net atmospheric pressure feeding the engine from 14.7 psi down to 11.8 to 10.35 psi.

We can calculate the effect of this on power. Flow, and therefore power, is proportional to the square root of pressure drop. If we take 14 psi as being representative of a good non restrictive manifold i.e. about 0.75 psi below atmospheric, then we’re looking at a pressure ratio of between 11.8/14 to 10.35/14 = 0.84 to 0.74.

Taking the square root of those we’re looking at a flow and power ratio of 0.92 to 0.86.

In other words we’re losing between 8% and 14% of the potential engine power. That means the 140 bhp should really be somewhere between 152 and 163 bhp. The average of those is pretty much exactly what I originally calculated the engine should produce with a good induction and exhaust system.

So as you can see, either the throttle is causing the restriction or the K-jet, it probably is the K-jet, if we look at the history and evolution of the Saab 99 normally aspirated rally car we can see that the competition department stopped using K-jet and moved over to twin carbs, i think i have just explained why. Although i have finished using the Bosch K-jet system and moved across to throttle bodies and ECU, more about this later, i am still interested to hear about other experiences with tuned Saab engines using K-jet.

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